This was useful because it meant your threaded jumps required fewer steps in the pipeline (compared to indexing a jump table), while also not needing any conditional for the threaded branch (e.g. not even checking for a NULL address before entering the loop, to initialize or substitute address placeholders).

The performance benefits of the constructor hack (low double digits, IIRC, maybe single digit, for a contrived test) were greater prior to Haswell (perhaps returning with Spectre mitigations?), but definitely not worth the headache of maintaining such non-portable, GCC-specific code. It did drive home, however, not only the usefulness of computed gotos (which are widely supported, excepting Visual Studio, and provide a far greater performance impact themselves), but the usefulness, in terms of design possibilities, of the ability to resolve computed goto addresses outside the function, which isn't supported anywhere except with that hack, AFAIK.